[CSI Generation and Reporting Operation]
In order to realize high-speed and large capacity communication between a radio communication base station apparatus (hereinafter, abbreviated as “base station”) and a radio communication terminal apparatus (hereinafter, abbreviated as “terminal (UE: User Equipment)”), 3GPP (3rd Generation Partnership Project) has standardized LTE (Long Term Evolution) and LTE-Advanced (hereinafter, abbreviated as “LTE-A”), and is currently carrying out the standardization for further enhancement.
LTE and LTE-A adopt OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme and adopt SC-FDMA (Single Carrier Frequency Division Multiple Access) as an uplink communication scheme.
For frequency scheduling and link adaptation in OFDMA, a base station instructs each terminal to report CSI (Channel State Information) and allocates appropriate resources using CSI. CSI is information including downlink channel quality (SINR or the like) measured using a desired signal component and an interference component. CSI includes CQI (Channel Quality Indicator), PMI (Precoding Matrix Indicator) and RI (Rank Indicator) which indicates a spatial multiplexing number.
LTE introduces two operations: operation of periodically reporting CSI (hereinafter referred to as “periodic reporting”) and operation of aperiodically reporting CSI (hereinafter referred to as “aperiodic reporting”). In periodic reporting, a terminal reports CSI by arranging the CSI on an uplink control channel which is a defined uplink resource at predetermined intervals. On the other hand, in aperiodic reporting, a terminal reports CSI using a resource on a common data channel at predetermined timing after receiving an instruction (or request) to report CSI from a base station. Aperiodic reporting is instructed to the terminal when an uplink data channel is allocated using downlink control channel PDCCH.
The terminal performs one measurement operation indicated in advance from the base station out of a plurality of measurement operations in one of periodic reporting and aperiodic reporting. This measurement operation is indicated from the base station to the terminal using a message for radio resource control (RRC signaling). Note that the base station can indicate different measurement operations for periodic reporting and aperiodic reporting to the terminal. For example, the base station may instruct the terminal to perform measurement operation to report RI, wideband PMI and wideband CQI in periodic reporting, and may instruct the terminal to perform measurement operation to report RI, wideband PMI, narrow band CQI in aperiodic reporting. In this case, periodic reporting may be used for rough link adaptation which is referenced when transmitting data with low urgency and aperiodic reporting may be used for detailed link adaptation which is referenced when transmitting data with high urgency.
The terminal measures channel quality of a measurement target indicated in advance from the base station and reports CSI. LTE-A Release10 has added an operation for a terminal to report CSI for two types of measurement targets indicated in advance in periodic reporting. In addition, LTE-A Release10 has added an operation for a terminal to associate two types of measurement targets indicated in advance with timing at which a base station instructs the terminal to report CSI and for the terminal to report CSI for one of the measurement targets in aperiodic reporting. The two types of measurement targets have different situations of interference components. The two types of measurement targets are indicated to the terminal from the base station using RRC with a bitmap corresponding to 40 continuous subframes.
LTE-A Release10 also has introduced a concept of CA (Carrier Aggregation) and increased the number of bits of trigger information (CSI request field) indicating aperiodic reporting from 1 bit to 2 bits. As a result, the base station is enabled to instruct the terminal to report CSI and report one of a plurality of CCs (Component Carriers) or a plurality of CSIs as the CSI to be reported by the terminal. For example, the base station indicates CSIs of two CCs as CSI1 and CSI2 using bits of trigger information, “00” indicating that “CSI is not reported,” “01” indicating that “only CSI1 is reported,” “10” indicating that “only CSI2 is reported” and “11” indicating that “CSI1 and CSI2 are reported” in advance through, for example, RRC signaling. The base station then transmits trigger information with one of the above-described combinations of bits to the terminal, and can thereby indicate aperiodic reporting and also indicate CSI to be reported.
Thus, LTE-A Release10 provides a mechanism of simultaneously reporting two CSIs with the introduction of CA. To put it more specifically, in aperiodic reporting, the terminal transmits two CSIs using an uplink data signal after a lapse of a predetermined time (4 subframes) after reception of CSI reporting from the base station.
[Coordinated Transmission by Plurality of Base Stations (CoMP)]
Meanwhile, in Release 11, which is the next version of LTE-A, CoMP (Coordinated Multi-Point) is being studied whereby a plurality of base stations cooperate to transmit signals to a terminal in a heterogeneous cell network (HetNet: Heterogeneous Network) in which there are a plurality of base stations having cover areas of different scales. HetNet is constructed of a macro cell (cell formed of a base station having large transmission power and coverage) and a pico cell (cell formed of a base station having small transmission power and coverage) within the macro cell. CoMP can reduce inter-cell interference and increase system throughput.
As CoMP schemes, 3GPP is studying mainly two CoMP schemes: (1) Coordinated beamforming (CB) and (2) Joint Transmission (JT).
CB assumes that data transmitted to a predetermined terminal is possessed by only one cell. In the case of CB, a terminal regards as interference, signals transmitted from a base station of a neighboring cell that possesses no data addressed to the terminal. CB adopts a technique of reducing inter-cell interference by controlling transmission parameters. Examples of specific transmission parameters include precoding, transmission power, modulation scheme and coding rate. By appropriately controlling these parameters, it is possible to weaken signals from the interfering cell addressed to the corresponding terminal while strengthening signals from a desired cell.
On the other hand, JT assumes that data transmitted to a predetermined terminal is possessed by a plurality of cells. Therefore, according to JT, respective base stations of the plurality of cells can simultaneously transmit signals to the corresponding terminal. Thus, unlike a conventional system, the terminal can handle a signal from a neighboring cell not as interference but as a desired signal, and therefore an SINR measured in the terminal is expected to improve. Especially by devising a method of generating precoding weights in a plurality of cells as operation within a network, it is possible to significantly improve the SINR measured in the terminal.
For CoMP control, LTE-A Release 11 provides a technique whereby a terminal measures channel quality of downlink to be controlled between a base station (hereinafter referred to as “coordinating base station”) of each cell and the terminal, and reports CSI to the coordinating base stations in cell units. To put it more specifically, in aperiodic reporting, the terminal transmits the same number of CSIs as the coordinating base stations using uplink data signals after a lapse of a predetermined time after reception of an instruction of CSI reporting from the coordinating base station.
[Reference Signal]
According to LTE-A Release11, a base station can indicate in advance, to a terminal, a resource to be measured to which a reference signal is mapped. As a reference signal, CSI-RS (Channel State Information-Reference Signal) is used. The base station indicates, to the terminal, information on a CSI-RS including a resource to which the CSI-RS is mapped using a message for radio resource control (RRC signaling) before the terminal performs CSI reporting.
FIGS. 1A to 1C are diagrams illustrating configuration examples of resources to which CSI-RSs are mapped. CSI-RSs are defined in 8-port, 4-port and 2-port configurations respectively according to the number of transmitting antenna ports of the base station. FIG. 1A illustrates a configuration example when the number of antenna ports is 8, FIG. 1B illustrates a configuration example when the number of antenna ports is 4, and FIG. 1C illustrates a configuration example when the number of antenna ports is 2. In FIGS. 1A to 1C, one subframe is made up of two resource blocks (RBs) each bundling 12 subcarriers. In FIGS. 1A to 1C, #i (0 to 19) represents a resource (2 REs (Resource Elements)) of two continuous OFDM symbols in the time domain within each subcarrier. In each resource (2 REs), CSI-RSs corresponding to two ports are code-multiplexed.
Each terminal acquires CSI-RS-related information from the base station in advance. Examples of the CSI-RS-related information include the number of antenna ports (Antenna Ports Count), CSI-RS configuration number (Resource Config: number #0 in FIG. 1 or the like) that identifies subcarriers within a subframe to which CSI-RS is mapped and OFDM symbol position, transmission subframe (Subframe Config) made up of transmission cycle and offset, and power ratio (p-C) between reference signals and data signals.
In FIGS. 1A to 1C, CSI-RS configuration numbers are assigned in order in the time direction and in order in the frequency direction at the same point in time. As shown in FIGS. 1A to 1C, the same number is assigned to the start position (start position in numbering order) of a resource of each CSI-RS configuration number among CSI-RS configurations corresponding to the respective numbers of antenna ports. As shown in FIGS. 1A to 1C, a CSI-RS configuration with a small number of antenna ports forms a subset of a CSI-RS configuration with a large number of antenna ports. The same number is assigned to resources with the same start position. This makes it possible to cover all resources to be identified with minimum necessary numbers for each number of ports while using overlapping numbers. For example, CSI-RS config(0) with 2 ports shown in FIG. 1C can be identified as only resources (2 REs) corresponding to two ports from the start position of CSI-RS config(0) (8 REs) with 8 ports shown in FIG. 1A. Resources indicated in this way are resources used to measure desired signal components (hereinafter referred to as “desired signal component measurement resources”) and called “non zero power CSI-RS resources.”
LTE-A Release11 also provides a muting technique that causes a coordinating base station in a connected cell to transmit a no transmission signal (signal with amplitude 0) to measure CSI-RS transmitted from a coordinating base station in a peripheral cell. To put it more specifically, of resources with configuration numbers #0 to #9 corresponding to four ports, resources to be designated as no transmission signals (resources to be muted) are indicated using a bitmap. Hereinafter, information indicating resources to be designated as no transmission signals will be referred to as “no transmission CSI-RS configuration number list (zeroTxPowerResourceConfigList).”
As an example, when configuration numbers (Resource Config) #1 and #2 are assumed to be no transmission signals, a no transmission CSI-RS configuration number list becomes {0, 1, 1, 0, 0, 0, 0, 0, 0, 0}. The base station indicates this no transmission CSI-RS configuration number list together with a transmission subframe (zeroTxPowerSubframeConfig) made up of a transmission cycle and offset to the terminal as in the case of the aforementioned CSI-RS, and the terminal can thereby identify resources which become no transmission signals in the corresponding subframe. The positions of the no transmission signals in the subframe corresponding to this example are #1 and #2 of CSI-RS shown in FIG. 2.
By associating a CSI-RS configuration of a coordinating base station of a peripheral cell with one CSI-RS configuration in the no transmission CSI-RS configuration number list, it is possible to avoid interference of a signal from the coordinating base station of the connected cell when measuring an SINR using the signal from the coordinating base station of the corresponding peripheral cell as a desired signal, and thereby improve accuracy of measuring CSI in the terminal. The resources indicated in this way are resources used to measure an interference component (hereinafter referred to as “interference component measurement resources”) and are called “zero power CSI-RS resources.”
Each resource of CSI-RS is used to measure one of a desired signal component and an interference component for one CSI report. The base station may indicate only a non zero power CSI-RS resource, indicate only a zero power CSI-RS resource or indicate a combination of a non zero power CSI-RS resource and a zero power CSI-RS resource. Note that the base station may instruct the terminal to use resources used to measure a desired signal component in one CSI report to also measure an interference component in another CSI report.
[Number of CSI Reports]
As described above, LTE-A Release10 provides the mechanism for simultaneously reporting two CSIs. On the other hand, a terminal simultaneously reports a plurality of CSIs to realize CoMP in LTE-A Release11 as well. In LTE-A Release11, unlike LTE-A Release10, the number of CSIs simultaneously generated is not limited to 2. For example, a base station of a macro cell, as a base station corresponding to a desired signal component, causes a terminal located in the vicinity of two pico cells within the macro cell area to measure SINRs of signals transmitted from three cells: a macro cell (TP-a), pico cell 1 (TP-b) and pico cell 2 (TP-c) and report the three CSIs. Thus, it is possible to transmit a signal from a base station having the best channel quality among the three cells, and thereby realize data transmission of high quality.
However, as the number of simultaneously reported CSIs increases, the amount of processing required to generate CSI for updating the information thereof increases. For example, in the case where the terminal is caused to simultaneously report three CSIs, the amount of processing required to generate CSIs becomes at least 1.5 times compared to a case where two CSIs are simultaneously generated. For this reason, the terminal may not be able to complete processing of CSI reporting within a predetermined time (4 subframes) in aperiodic reporting. Thus, a technique for handling this increase in the amount of processing is necessary.
A first conventional technique for handling the increase in the amount of processing may be to limit to a maximum of 2, the number of CSIs to be reported in one reporting unit of aperiodic reporting which is indicated by the base station using RRC signaling. It is thereby possible to make the amount of processing required to generate CSI equivalent to that in the related art.
Moreover, as a second conventional technique for handling the increase in the amount of processing may be to lessen the allowable value of the processing time (see NPL 1). To put it more specifically, although CSI is conventionally reported after a lapse of 4 subframes after reception of an instruction of CSI reporting, CSI may be reported after a lapse of 6 subframes. This makes it possible to complete processing of CSI reporting without increasing the amount of processing per unit time.